1. Field of the Invention
The present invention is concerned with the recovery of a protein expressed by open reading frame 2 of porcine circovirus type 2. More particularly, the protein is a recombinant protein expressed by a transfected virus containing recombinant coding sequences for porcine circovirus type 2, open reading frame 2. Still more particularly, the transfected virus is permitted to infect cells in growth media and the protein expressed by open reading frame 2 is recovered in the supernate, rather than from inside the cells. Even more particularly, the method involves the steps of amplifying the open reading frame 2 gene from porcine circovirus type 2, cloning this amplified portion into a first vector, excising the open reading frame 2 portion from this first vector and cloning it into a transfer vector, cotransfecting the transfer vector with a viral vector into cells in growth media, causing the cells to become infected by the viral vector and thereby express open reading frame 2, and recovering the expressed recombinant protein coded for by open reading frame 2 in the supernate.
2. Description of the Prior Art
Porcine circovirus type 2 (PCV2) is a small (17-22 nm in diameter), icoshedral, non-enveloped DNA virus, which contains a single-stranded circular genome. PCV2 shares approximately 80% sequence identity with procine circovirus type 1 (PCV1). However, in contrast with PCV1, which is generally non-virulent, swine infected with PCV2 exhibit a syndrome commonly referred to as Post-weaning Multisystemic Wasting Syndrome (PMWS). PMWS is clinically characterized by wasting, paleness of the skin, unthriftiness, respiratory distress, diarrhea, icterus, and jaundice. In some affected swine, a combination of all symptoms will be apparent while other swine will only have one or two of these symptoms. During necropsy, microscopic and macroscopic lesions also appear on multiple tissues and organs with lymphoid organs being the most common site for lesions. A strong correlation has been observed between the amount of PCV2 nucleic acid or antigen and the severity of microscopic lymphoid lesions. Mortality rates for swine infected with PCV2 can approach 80%. In addition to PMWS, PCV2 has been associated with several other infections including pseudorabies, porcine reproductive and respiratory syndrome (PRRS), Glasser's disease, streptococcal meningitis, salmonellosis, postweaning colibacillosis, dietetic hepatosis, and suppurative bronchopneumonia.
Open reading frame 2 (ORF2) protein of PCV2, having an approximate molecular weight of 30 kDa when run on SDS-PAGE gel, has been utilized in the past as an antigenic component in vaccines for PCV2. Typical methods of obtaining ORF2 for use in such vaccines generally consist of amplifying the PCV2 DNA coding for ORF2, transfecting a viral vector with the ORF2 DNA, infecting cells with the viral vector containing the ORF2 DNA, permitting the virus to express ORF2 protein within the cell, and extracting the ORF2 protein from the cell via cell lysis. These procedures generally take up to about four days after infection of the cells by the viral vector. However, these procedures suffer because the extraction procedures are both costly and time-consuming. Additionally, the amount of ORF2 recovered from the cells is not very high; consequently, a large number of cells need to be infected by a large number of viral vectors in order to obtain sufficient quantities of the recombinant expressed protein for use in vaccines and the like. Accordingly, what is needed in the art is a method of obtaining ORF2 protein which does not require extraction of the ORF2 protein from within infected cells. What is further needed are methods of obtaining recombinant ORF2 protein sufficient for efficiently preparing vaccine compositions. Finally, what is needed are methods of obtaining ORF2 protein which do not require the complicated and labor-intensive methods required by the current ORF2 protein extraction protocols.